1. FIELD OF THE INVENTION
The present invention relates to an apparatus for generating an output signal having a frequency proportional to the road speed of a vehicle.
2. DESCRIPTION OF THE PRIOR ART
In recent years, the factors of safety, environmental concern and convenience have created a demand for vehicle speed control devices. For example, in an automobile, the driver's attention must be divided between watching the traffic and road and watching the speedometer so that he can maintain a chosen speed. In addition, on a long trip it becomes quite tiring to manually control the accelerator pedal since the driver's right foot and leg must remain in relatively the same position. When a speed control apparatus is utilized, the driver is free to be constantly alert to the traffic and road conditions and will arrive at his destination in a less tired condition. Furthermore, the maintenance of a constant speed tends to increase gas mileage and decrease automobile emissions which are important environmental goals.
Today, many trucks include power take-off units for driving auxiliary equipment. Often it is desirable to maintain a uniform operating speed under varying load conditions imposed on the truck engine by the auxiliary equipment. Normally, this requires an operator who must control the accelerator pedal in response to the engine speed as read from a tachometer. This is a tiring and a difficult job and often one or more other workers must be utilized to monitor and/or operate the auxiliary equipment. Therefore, a speed control apparatus may be utilized to advantage to control the engine at a uniform speed. Such operation tends to reduce fuel consumption and engine emissions and may allow a reduction in the number of workers required.
The speed control apparatus requires as an input a signal representing the actual speed to be controlled. In previous speed control systems, it has been common practice to derive the actual speed signal from the speedometer cable. This is not a difficult task when the vehicle is being constructed since the required connection to some form of actual speed signal generating means may be provided. However, it is much more difficult to add a speed control apparatus to an existing vehicle. The speedometer cable must be replaced by a speedometer cable modified to drive a speed signal source. This requires the production of a great many speedometer cables for use with the various models of cars on the road today. Such an approach, therefore, is costly from the standpoint of the large inventory required and the large amount of time required to replace the speedometer cables.